U.S. patent number 4,450,064 [Application Number 06/477,098] was granted by the patent office on 1984-05-22 for electrochemical gas sensor and method for producing the same.
This patent grant is currently assigned to Beckman Instruments, Inc.. Invention is credited to John N. Harman, III.
United States Patent |
4,450,064 |
Harman, III |
May 22, 1984 |
Electrochemical gas sensor and method for producing the same
Abstract
An improved method for producing an electrode assembly for use
in electrochemical gas sensors. A plurality of conductors are
attached to one surface of a generally circular electrode blank.
These conductors are passed through respective conductor routing
holes in an electrically nonconductive mounting member. After the
electrode blank is attached to the mounting member, predetermined
sections of the electrode blank are cut away to divide the blank
into a plurality of electrically isolated regions that are
connected to respective conductors. The cut-away sections may then
be filled with an electrically nonconductive filling material to
provide the electrode assembly with a smooth, flush surface.
Inventors: |
Harman, III; John N.
(Placentia, CA) |
Assignee: |
Beckman Instruments, Inc.
(Fullerton, CA)
|
Family
ID: |
23894534 |
Appl.
No.: |
06/477,098 |
Filed: |
March 21, 1983 |
Current U.S.
Class: |
204/412; 204/415;
29/619; 600/355 |
Current CPC
Class: |
G01N
27/404 (20130101); Y10T 29/49098 (20150115) |
Current International
Class: |
G01N
27/49 (20060101); G01N 027/46 () |
Field of
Search: |
;204/412,415,416,418,1T,1Y,1P ;128/635 ;29/61R,619,621,825,854 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Phelan, et al., "A Maintenance Free Dissolved Oxygen Monitor",
American Lab., Jul. 1982, pp. 65-72..
|
Primary Examiner: Kaplan; G. L.
Assistant Examiner: Nguyen; Nam X.
Attorney, Agent or Firm: Steinmeyer; R. J. Harder; P. R.
Jason; E. C.
Claims
What is claimed is:
1. An improved method for producing an electrode assembly that
includes a plurality of electrodes comprising:
(a) forming an electrically nonconductive mounting member having a
plurality of conductor routing holes,
(b) forming an electrode blank from an electrically conductive
material,
(c) attaching a plurality of conductors to predetermined points on
one surface of the electrode blank,
(d) passing said conductors through respective ones of said
conductor routing holes,
(e) attaching the electrode blank to the mounting member, and
(f) cutting away sections of the electrode blank to partition the
same into electrically isolated regions that are attached to
different respective conductors.
2. The method of claim 1 including the further step of filling in
said cutaway sections with an electrically nonconductive filling
material.
3. The method of claim 2 including the further step of trimming
said filling material to form a smooth surface that is flush with
said isolated regions.
4. The method of claim 1 in which the mounting member and the
electrode blank are generally circular, and in which the cutting
step comprises the cutting away of an annular section of the
electrode blank.
5. The method of claim 1 in which the attaching step comprises the
cementing of the electrode blank to the mounting member.
6. The method of claim 1 in which the cutting step divides the
electrode blank into a plurality of generally circular electrodes
which are concentric with one another.
7. An improved electrochemical gas sensor of the type having a gas
permeable membrane, a housing defining an electrolyte chamber for
receiving an electrolyte solution, and a mounting member for
mounting a plurality of electrodes in the electrolyte chamber in
the vicinity of the gas permeable membrane, said gas sensor being
produced by:
(a) providing a plurality of conductor routing holes through the
mounting member,
(b) providing an electrode blank having a thickness that is small
in relation to its length and width,
(c) electrically connecting a plurality of conductors to one
surface of the electrode blank,
(d) passing the conductors through respective ones of said routing
holes,
(e) attaching the electrode blank to the mounting member, and
(f) cutting said blank into electrically isolated regions that are
attached to different conductors.
8. The gas sensor of claim 7 in which the electrode blank is
generally circular and in which the cutting step cuts away
generally annular shaped sections of the electrode blank.
9. The gas sensor of claim 8 produced by the further step of
filling in the cutaway sections of the electrode blank with an
electrically nonconductive filling material.
10. The gas sensor of claim 9 including the further step of
trimming said filling material to form a smooth surface that is
flush with said isolated regions.
11. The gas sensor of claim 7 in which the electrode blank is
generally circular and in which the cutting step divides the blank
into a plurality of electrodes which are concentric with one
another.
12. The gas sensor of claim 11 including a disc-shaped inner
electrode and at least one generally annular shaped outer
electrode.
13. The gas sensor of claim 7 including a plurality of generally
annular shaped electrodes.
14. The gas sensor of claim 7 in which the mounting member has a
generally cylindrical shape and is located at approximately the
center of the electrolyte chamber.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing
electrochemical gas sensors and is directed more particularly to a
method for producing electrochemical gas sensors that include a
plurality of closely spaced electrodes.
In measuring the concentrations of electrochemically reducible
gases such as oxygen, it is a common practice to utilize
membrane-type gas sensors. Sensors of the latter type include a
gas-permeable membrane that is located at one end of a chamber
which is filled with a suitable electrolyte solution, such as an
aqueous solution of potassium chloride. Immersed in the electrolyte
solution is an anode electrode and a cathode electrode. Of these,
the cathode electrode is usually located adjacent to the membrane
to maximize its exposure to the gas to be measured. These
electrodes are connected to a remote instrument console which
applies a polarizing voltage to the electrodes and provides a
user-readable indication of the current that flows
therebetween.
When oxygen is being measured, for example, oxygen molecules
diffuse through the membrane and are reduced to hydroxyl ions in
the layer of electrolyte that lies between the membrane and the
cathode electrode. As this occurs, a corresponding oxidation
reaction occurs at the anode electrode. As the oxygen in this layer
of electrolyte is reduced, an oxygen concentration gradient is
established between it and the main body of the electrolyte. This
concentration gradient, in turn, causes any oxygen that is
dissolved in the main body of electrolyte to diffuse toward the
cathode electrode. As this dissolved oxygen arrives at the cathode
it too is reduced, thereby giving rise to an error in the measured
oxygen concentration. The part of the sensor current which is
attributable to these diffusing gas molecules represents one
component of the residual current of the sensor.
In order to reduce the effect of such residual currents, some gas
sensors are provided with guard electrodes which serve to reduce
diffusing gas molecules before they arrive at the cathode. One
guard electrode of this type is shown in U.S. Pat. No. 3,454,485,
issued in the name of Hauk et al., on July 8, 1969.
Guard electrodes have also been used in transcutaneous oxygen
sensors. An example of a sensor of the latter type is shown in U.S.
Pat. No. 4,324,257, issued in the name of Albarda et al., on Apr.
13, 1982.
While gas sensors having guard electrodes operate satisfactorily,
they are expensive to produce. The reason is that guard electrodes
and the mounting structures that are used to support them are
bulky. As a result, gas sensors that are to include guard
electrodes may have to be totally redesigned to accommodate them.
This, in turn, may involve the cost of making new injection molds
and/or extensive machining.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided an
improved method for producing a gas sensor having a guard
electrode. Generally speaking, the present invention makes use of
an electrode mounting member having a plurality of holes, and an
electrode blank to which a plurality of conductors have been
attached. After passing the conductors through respective holes in
the mounting member, the electrode blank is permanently attached to
the mounting member. Annular sections of the electrode blank are
then cut away to partition the electrode blank into a plurality of
electrically isolated regions each of which is connected to a
respective conductor. These cutaway sections may then be filled
with a suitable electrically nonconductive filling material to
produce a smooth, flush surface that includes a plurality of
closely spaced electrodes.
In the preferred embodiment, the electrode blank is partitioned
into a generally circular inner electrode and a generally annular
outer electrode that is concentric therewith. More generally,
however, the method of the invention may be used to partition the
electrode blank into three or even more electrodes having a variety
of different physical configurations. The invention therefore makes
possible the fabrication of electrodes which may be used in a
variety of different operating modes.
These and other objects of the present invention will become
apparent from the following description and drawings in which:
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a gas sensor of a type that is
known in the art,
FIG. 2 is an enlarged cross-sectional view of the cathode mounting
assembly of the sensor of FIG. 1,
FIGS. 3, 7 and 8 are enlarged cross-sectional views of electrode
assemblies that have been produced by the practice of the present
invention,
FIG. 4 shows the appearance of the electrode of FIG. 3 prior to the
final assembly thereof, and
FIGS. 5 and 6 are simplified schematic diagrams which show the
electrical connections used with two different gas sensors that are
constructed in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 there is shown a simplified cross-sectional
view of a gas sensor of a type that is known in the art. This
sensor includes upper and lower housing sections 10 and 12,
respectively, which may be made of a suitable plastic material such
as polyvinyl chloride. Housing sections 10 and 12 enclose a
generally cylindrical electrolyte chamber 14 which is filled with a
suitable electrolyte solution such as an aqueous solution of
potassium chloride. Located within chamber 14 is an anode electrode
16 which comprises a helically wound sheet of a metal such as
silver. Electrode 16 may be connected to a suitable polarizing
voltage source of a known type (not shown) through a conductor 18.
Also located within chamber 14 is a cathode assembly which includes
a generally circular cathode electrode 20 that is composed of a
noble metal such as gold. The cathode assembly also includes a
cathode mounting post 22 and a conductor 24 through which cathode
20 may be connected in closed circuit relationship with the
above-mentioned polarizing voltage source.
Closing the lower end of electrolyte chamber 14 is a gas-permeable
membrane 26 that typically comprises a thin sheet of
polytetrafluoroethylene which is supported in close proximity to
cathode 20. This membrane may be mounted in any suitable manner,
such as by being sealed between the two halves 28a and 28b of a
compression ring assembly which is, in turn, sealed against the
lower end of housing 10 by an O-ring 30 when housing section 12 is
tightened against housing section 10.
The gas to be measured is supplied to cathode 20 through an opening
32 in lower housing section 12. The component of interest, which
may for example be oxygen, diffuses through membrane 26 and into
the thin layer of electrolyte that is present between cathode 20
and membrane 26. Once in this thin layer of electrolyte, the oxygen
molecules are reduced to hydroxyl ions by electrons received from
cathode 20. As this occurs, an equal number of electrons are
supplied by the oxidation of silver at anode 16. The current that
flows in conductors 18 and 24 as a result of this reaction is
therefore a direct measure of the quantity of oxygen that is
present. This current is usually measured by means of a remote
current measuring instrument (not shown).
Referring to FIG. 2, there is shown an enlarged cross-sectional
view of the end of the cathode assembly of FIG. 1. This enlarged
view makes clear that cathode 20 rests in a shallow annular recess
34 at the end of mounting member 22. During assembly, cathode
electrode 20 is first bonded to wire 24 by a quantity of a suitable
solder 36. Then, after applying a quantity of a suitable
electrochemically inert cementitious material such as an epoxy
resin to recess 34, the cathode and its attached conductor are
pushed into an opening 23 through the center of mounting element 22
until cathode 20 comes to rest in recess 34. After this material
has cured, internal opening 23 may be filled with a suitable
potting compound prior to the assembly of the remainder of the
sensor.
Because the operation of the sensor of FIG. 1 consumes oxygen, the
layer of electrolyte between cathode 20 and membrane 26 will
quickly become depleted in dissolved oxygen. This depletion causes
an oxygen concentration gradient to appear between the electrolyte
layer and the main body of electrolyte in chamber 14. This
gradient, in turn, causes any oxygen that is dissolved in the main
body of electrolyte to diffuse toward cathode 20. As this diffusing
oxygen enters the electrolyte layer, it too is reduced and thereby
gives rise to a current between anode 16 and cathode 20. Since this
current bears no relationship to the oxygen concentration outside
of membrane 26, it represents an error in the desired oxygen
concentration measurement.
As explained in the above-mentioned Hauk et al. patent, the above
error may be reduced by providing a guard electrode which surrounds
the cathode and consumes any dissolved oxygen which diffuses toward
it from chamber 14. This reduction occurs because the current
through the guard electrode does not flow through cathode 20 and
does not therefore register on the instrument which measures the
current between electrodes 16 and 20.
While known types of guard electrodes operate well, they
substantially increase the cost of the gas sensors that incorporate
them. This is not only because of the cost of the guard electrode
and its mounting elements, but also because the gas sensor may have
to be entirely redesigned to accommodate these additional elements.
Through the use of the present invention, the cost of providing a
guard electrode is greatly reduced and the need to change the
overall design of the sensor is substantially eliminated. The
manner in which this is accomplished is most easily understood with
reference to FIGS. 3 and 4.
Referring to FIG. 3, there is shown an enlarged partial
cross-sectional view of an electrode assembly which has been
produced in accordance with the present invention. This electrode
assembly includes an electrically nonconductive mounting member
22', a generally disc-shaped cathode or measuring electrode 20' and
a generally annular shaped guard electrode 25. Electrode 20' is
connected to the polarizing voltage source and the current
measuring instrument through a conductor 24 which passes through a
first, centrally located conductor routing hole 23' in mounting
member 22'. Guard electrode 25 is connected to the polarizing
voltage source through a conductor 27 which passes through a
second, off-center conductor routing hole 29 in mounting member
22'. The connections of these conductors to the polarizing voltage
source and the current measuring instrument may be as shown in FIG.
5.
Electrodes 20' and 25 are preferably cemented into a recess 35
which is cut into the end of member 22' and has a shape that
matches that of the inner surfaces of electrodes 20' and 25. The
open spaces or gaps between electrodes 20' and 25 are preferably
filled with a body 36 of a suitable electrochemically inert filling
material such as an epoxy resin. This filling material allows the
end of the electrode assembly to present a smooth rounded surface
to the membrane with which it will operate.
The manner in which the electrode assembly of FIG. 3 is made is
most easily understood with reference to FIG. 4, which shows an
exploded view of the parts that are used to make it. Included in
this exploded view is an electrode blank 38 which comprises a
generally disc-shaped piece of metal having a curvature that is
substantially the same as that of recess 35. Before electrode blank
38 is attached to mounting member 22', conductors 24 and 27 are
soldered to the inner surface thereof at points that allow these
conductors to be aligned with conductor routing holes 23' and
29.
After mounting member 22' and electrode blank 38 have been
completed, the following steps are performed in completing the
electrode assembly of FIG. 3. First, conductors 24 and 27 are
passed through respective ones of the conductor routing holes in
member 22'. Second, electrode blank 38 is securely attached to
member 22'. A suitable electrochemically inert cementitious
material such as an epoxy resin may, for example, be placed in
recess 35 prior to time that electrode blank 38 is inserted
therein. Blank 38 may, however, be attached to member 22' in any
manner which will securely hold both the center and edge thereof
against member 22'.
After blank 38 has been attached to member 22', a cutting tool such
as a lathe is used to cut away an annular section of electrode
blank 38 and thereby partition the same into two concentric
electrically isolated regions that are connected to conductors 24
and 27, respectively. These two regions correspond to the
electrodes labeled 20' and 25 in FIG. 3. The open space or gap
between these regions may then, if desired, be filled with a body
36 of the previously mentioned filling material and then trimmed so
that the end of the electrode assembly has a smooth flush surface.
The gas sensor may then be completed in the usual manner by adding
the remaining parts shown in FIG. 1.
In view of the foregoing it will be seen that the machining that is
necessary to accommodate the guard electrode is confined to the
interior and end of mounting member 22'. Since the interiors and
ends of known mounting members also have to be machined, it is
apparent that the machining necessary to accommodate the guard
elecrtrode does not substantially increase the cost of producing
the gas sensor. Moreover, the inclusion of the guard electrode does
not require any changes in the molds that are used to produce
housing sections 10 and 12. Thus, the practice of the present
invention not only produces the desired guard electrode, but does
so in a manner that does not significantly change the overall
design of the gas sensor or increase the cost of producing the
same.
The operation of electrodes 20' and 25 as measuring and guard
electrodes will now be discussed with reference to the simplified
schematic diagram shown in FIG. 5. As shown in FIG. 5, anode 16 and
cathode 20' are connected substantially in series between a source
of polarizing voltage V and one input of an operational amplifier
A. Since the output current of amplifier A flows through an ammeter
M, meter M provides a visual indication of the magnitude of the
current flow between the anode and cathode electrodes. Since the
latter current is dependent upon the quantity of oxygen which
diffuses through membrane 26, the magnitude of the ammeter current
can be used as a direct measure of the oxygen concentration in the
vicinity of membrane 26.
Unlike measuring electrode 20', guard electrode 25 is connected in
series with polarizing voltage source V through a path which does
not include the input of amplifier A. As a result, guard electrode
25 is able to consume any dissolved oxygen molecules that appear in
the adjacent electrolyte without producing any current in amplifier
A. It is therefore able to prevent these oxygen molecules from
affecting the flow of current through cathode 20'. Thus, guard
electrode 25 prevents meter M from displaying the residual current
of the gas sensor.
In addition to being usable in the fabrication of gas sensors that
include a guard electrode, the present invention may also be used
in the fabrication of gas sensors that operate in the
potentiostatic mode. A simplified schematic drawing of a gas sensor
of the latter type is shown in FIG. 6. In the latter figure the gas
sensor includes a cathode electrode 20', an anode electrode 25 and
a reference electrode R. Because cathode 20' is connected in closed
circuit relationship with reference electrode R, a polarizing
voltage source V and the input of amplifier A, the voltage between
electrode R and electrode 20' tends to remain substantially
constant. As oxygen is reduced at cathode 20', the potential of
cathode 20' tends to change, causing amplifier A to produce between
electrodes 25 and 20' a current which is just sufficient to force
the voltage between the cathode and reference electrodes to return
to its original value. Since the magnitude of the latter current is
dependent upon the quantity of oxygen that diffuses through
membrane 26, the current through meter M can be used as a direct
measure of the oxygen concentration in the vicinity of membrane 26.
It will therefore be seen that the present invention may be used to
fabricate of gas sensors that operate in a variety of different gas
detection modes.
The method of the present invention may also be used in fabricating
gas sensors that have more than two electrodes or that have
unconventional electrode configurations. Referring to FIG. 7, for
example, there is shown an electrode assembly which includes a
generally disc-shaped central electrode 20' which is surrounded by
two generally annular shaped electrodes 25 and 40. This
three-electrode embodiment is produced in generally the same manner
as the two-electrode embodiment of FIG. 3. In particular, mounting
member 22" is provided with conductor routing holes 23', 29 and 43
through which conductors such as 24, 27 and 42 may be passed. After
these conductors are passed through respective holes and the
electrode blank is attached to the end of mounting member 22"
sections of the electrode blank are cut away to partition the
electrode blank into three electrically isolated regions that are
connected to respective conductors. The resulting open spaces or
gaps may then be filled with the above-mentioned filling material
and trimmed to provide a smooth flush surface at the end of the
electrode assembly. It will therefore be seen that, provided that
the diameter of the electrode blank and mounting element are made
large enough, there is no limit to the number of electrodes that
may be produced by the practice of the present invention.
Referring to FIG. 8, there is shown an electrode assembly which is
generally similar to that of FIG. 7, except that it does not
include a disc-shaped central electrode. An assembly having this
configuration may be produced in generally the same manner as the
assembly of FIG. 7, except for the additional steps of cutting away
the central portion of the electrode blank and filling the
resulting opening with a suitable filling material. Alternatively,
the electrode assembly of FIG. 8 may be produced by using an
electrode blank that has a hole in its center. This central hole
may then be filled, as before, or may engage a suitable central
projection from mounting member 22"'. Electrode assemblies of the
type shown in FIG. 8 are adapted to operate in gas sensors in which
the electrodes are preferably relatively narrow and closely spaced.
One gas sensor of the latter type is shown and described in U.S.
Pat. No. 4,076,596, which issued on Feb. 28, 1978 in the name of
Connery et al.
In view of the foregoing, it will be seen that the method of the
present invention comprises a simple and inexpensive method for
producing electrode assemblies that include a plurality of
electrodes which are arranged in any of a variety of different
configurations. In addition, because of the way that these
electrodes are produced, the present invention lends itself to
producing a plurality of electrodes in the same space that had once
been occupied by only a single electrode. As a result, the benefits
of multi-electrode configurations may be incorporated into existing
gas sensors without having to make basic changes in their
design.
* * * * *